1. Field of the Invention
The present invention relates to a measles virus mutant antigen and a gene coding for the same. More particularly, the present invention is concerned with a measles virus mutant antigen comprising at least one protein antigen selected from the group consisting of a measles virus mutant H protein antigen and a measles virus mutant F protein antigen, and a measles virus mutant gene coding for the measles virus mutant antigen. By the use of the measles virus mutant antigen or the gene coding for the same of the present invention, it has become possible to provide efficiently and economically a live attenuated measles vaccine. or gene vaccine which is adapted for an epidemic strain of measles virus, and a diagnostic reagent capable of accurately detecting infections with an epidemic strain of measles virus.
2. Prior Art
(1) Pathogenicity: Measles virus is the pathogen of measles, and it is distributed widely throughout the world. This virus is highly infectious, and when a person suffers a droplet infection with a measles virus, damage occurs mainly in the respiratory system and reticuloendothelial tissue, thereby causing an acute disease. A person suffering from measles shows systemic symptoms, such as high fever, catarrh and rash. Further, in severe cases, measles is complicated with bacterial pneumonia, tympanitis and acute encephalitis. In 1996, the number of measles patients and number of deaths due to measles in the world were estimated to be about 42 million and about 1 million 10 thousand, respectively ["The World Health Report 1997", p. 15, WHO (World Health Organization) published in 1997]. As apparent from the above, measles is an infectious disease which should be taken into serious consideration, and eradication of measles by vaccines is desired throughout the world. In this situation, the Expanded Program on Immunization (EPI) of World Health Organization (WHO) has already adopted a measles eradication program with the goal of controlling measles by the year 2010.
(2) Viral morphology and genomic structure: According to the Sixth Report of the International Committee on Taxonomy of Viruses, the measles virus is classified under the order Mononegavirales, family Paramyxoviridae, genus Morbillivirus. The virion of the measles virus is generally spherical (diameter: about 150 nm) and has an envelope composed of a lipid bilayer. On the surface of the envelope are spike-like projections composed of an H (hemagglutinin) protein and composed of an F (fusion) protein, and the bases of the projections (proteins) are supported by a matrix membrane protein at the inner layer of the envelope. The nucleocapsid present in the inside of the envelope consists of measles virus genomic RNA which is a linear, non-segmented (-) sense (that is, mononega) RNA having a length of about 16 kb, and proteins. The genomic RNA codes for N (nucleocapsid-associated proteins), P/C/V (phosphoprotein/C protein/V protein: coded for by tricistronic gene), M (matrix protein), F (fusion protein), H (hemagglutinin protein) and L (large putative polymerase protein), and the coding regions are located in this order from the 3' end to the 5' end of the genome ("Virus Taxonomy: Sixth Report of the International Committee on Taxonomy of Viruses", Archives of Virology, Supplement 10, pp. 268-270 and pp. 271-272, 1995).
(3) Conventional virus strain for live attenuated measles vaccine: Examples of the virus strains known for live attenuated measles vaccine are: CAM-70, Schwarz FF8, AIK-C, AIK-HDC, TD97, Moraten, Connaught, Schwarz, Edmonston B, Edmonston-Zagreb, Leningrad-16, Shanghai-191, Changchum-47 and Beijing (S. A. Plotokin and E. A. Mortimer, "Vaccines", 2nd edition, pp. 238-239, published by W. B. Saunders Company, 1994). These virus strains are either a host-range mutant or a temperature mutant of measles virus which are attenuated to ensure safety and effectiveness so as to be used as an active component for a live vaccine, and such viruses are obtained by sequentially subjecting an isolated strain (wild measles virus) to passages of culture under different conditions prepared by combining various factors, such as host cell, culture temperature, and pH and composition of a culture medium.
(4) Prevention: Vaccines for preventing measles were put to practical use in the early 1960's. At the beginning, the majority of the measles vaccines used was killed (or inactivated) vaccines (abbreviated "K") containing killed measles viruses as an active component of the vaccine. However, the killed measles vaccine had an unsatisfactory immunological effect, and further, it induced serious atypical measles. In this situation, the use of a live vaccine (abbreviated "L") containing live attenuated measles viruses as an active component of the vaccine gradually became predominant in the late 1960's. A combination of K and L vaccines was adopted, but since the 1970's, a further attenuated live vaccine (abbreviated "FL") obtained by further attenuating the above-mentioned live vaccine virus has become commercially available throughout the world for practical use. With respect to the live vaccine, each of the live attenuated measles vaccine strains mentioned in item (3) above is used as an active component of the vaccine.
(5) Problems of measles vaccine and diagnosis: With respect to the maintenance of immunity obtained by using a conventional live attenuated measles virus vaccine, some problems have arisen since the early 1970's. Illustratively stated, reports on secondary vaccine failure and modified measles have been made, in which it is reported that, people who have been vaccinated with measles vaccine were reinfected with measles and suffered from symptoms which are different from that of the natural infection (in general, the symptoms are mild compared to those of the natural infection, but serious in rare cases). Such reports on reinfection in various parts of the world were made sporadically in the latter half of the 1980's, and the reports are frequently made in the 1990's. Therefore, the development of means for preventing the reinfection and for determining the infecting virus has been earnestly desired by not only the people in various countries of the world, but also by the WHO from the viewpoint of the above-mentioned eradication program on measles. However, a measles vaccine or diagnostic reagent effective for preventing the infection with the currently prevailing measles viruses has not yet been realized.